1. FIELD OF THE INVENTION
The present invention relates to a magnetic recording medium, and more particularly to a perpendicularly magnetizable magnetic recording medium for super high density recording.
2. DESCRIPTION OF THE PRIOR ART
Extensive researches are being made by various circles on the perpendicularly magnetizable magnetic recording system as a prospective system for super high density recording expected to be commercially available in near future. In the conventional magnetic recording media, magnetic recording is conducted in such a manner that a magnetic layer formed on a substrate is magnetized in a horizontal direction of the magnetic layer (i.e. in-plane magnetization). When the recording density is increased in the horizontal magnetization recording, the diamagnetic field of the magnetic layer increases, which in turn leads to a reduction and rotation of the residual magnetization and consequently the reproducing output will considerably be decreased. Thus, there is a practical limitation in the increase of the recording density. Under the circumstances, a perpendicular magnetization recording system in which magnetization is conducted in a direction perpendicular to the surface of the magnetic layer (i.e. in a direction of the thickness of the magnetic layer), has been proposed as a system which breaks through the limitation in the increase of the recording density of the horizontal magnetization recording system. In the perpendicular magnetization recording, the shorter the wave length of the recording signals is i.e. the greater the recording density is, the smaller the demagnetization field becomes. Accordingly, the perpendicular magnetization recording system is expected to be more advantageous for the high density recording than the horizontal magnetization recording system.
As a recording medium useful for such a perpendicular magnetization recording system, a Co-Cr layer formed by a sputtering method has already been proposed as being extremely effective. The sputtering method is well known as one of the methods for the preparation of thin magnetic metal layers, in which when glow discharge is conducted in a vacuumed inert gas atmosphere, the gas ions are driven to collide with the cathode (i.e. the target) to evaporate the cathode material in a form of atoms or groups of atoms and the sputtered metal atoms are deposited on the surface of the substrate disposed close to the anode.
The present inventors have conducted researches on this Co-Cr layer with a view to its practical applications and mass production, and during the course of the researches, they have realized that there are some practical problems involved. Namely, firstly, when a high speed layer-forming operation is carried out by means of a sputtering device, the coercive force (Hc (.perp.)) tends to be so great that no adequate overwrite characteristic is thereby obtainable in the actual digital recording, and secondly, the coercive force (Hc (.perp.)) is highly dependent on the temperature of the substrate at the time of the layer-forming operation, and accordingly if the layer-forming operation is continuously carried out for mass production, the substrate temperature is unavoidably varied by the radiation heat from the target and the coercive force (Hc (.perp.)) is correspondingly changed, whereby it becomes impossible to form a layer having a uniform quality.
Firstly, the overwrite characteristic must be at least 26 dB. To obtain the overwrite characteristic of such a level, the coercive force (Hc (.perp.)) must be at most 600 Oe. In the case of the Co-Cr layer, when the layer-forming operation is conducted at a low deposition speed (i.e. less than several hundreds .ANG./min) in an experimental laboratory scale, it is possible to obtain a coercive force (Hc (.perp.)) of less than 600 Oe by properly cooling the substrate. However, in an industrially feasible high speed layer-forming operation where the deposition speed is several thousands .ANG./min. the coercive force inevitably becomes to be at least 600 Oe, and in an extremely case, it reaches a level as high as 1,800 Oe. Thus, in the case of the Co-Cr layer, it is practically impossible to obtain an adequate overwrite characteristic for actual digital recording at a production speed feasible for an industrial operation.
Secondly, with respect to the dependency of the coercive force (Hc .perp.)) on the substrate temperature, it has been found that in the case of Co-Cr layer, the coercive force increases with an increase of the substrate temperature at a rate of at least 30 Oe/10.degree. C. The substrate is heated mainly by the radiation heat from the target, and in the case of continuous production, it is expected that the substrate temperature changes at every moment and it is impossible to obtain a layer having a uniform characteristic so long as the dependency of the coercive force on the substrate temperature is so great as mentioned above.
In this technical field, there has been an attempt to incorporate a third component such as rhodium to the Co-Cr system, but the above mentioned problems have not thereby been solved.